Is it valid to use the NULL constant with cout ?
 Ex:  cout << NULL  ;
I have seen that this has the  same effect as
      cout << ""   ;
on all the  C++  compilers  I have used.

Or could this be a *bug*  in  these  compilers ??

Thanx.

Govind
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In article <1992Feb13.182905.24251@uwm.edu> govind@miller.cs.uwm.edu (Govind Seshadri) writes:
>Is it valid to use the NULL constant with cout ?
> Ex:  cout << NULL  ;

The symbol `NULL' is not well defined in the manner you wish to use it.
The only `sure' way to check a pointer against the null pointer is to
check it against `0' (the FAQ has more to say about this issue).  So,
if NULL is #define'd to be the token `0', cout will think it is printing
the integer zero, and will print "0".  If NULL is #defined to be ((void*)0),
ostream::operator<<(const void*) will get control which prints out the
pointer as an address, usually in hex, and "0x00000000" or "0" will be
printed.  If NULL is #defined as ((char*)0), or if there is no overload
for printing void*'s via ostream, cout may try to print this as a '\0'
terminated character string, and mayhem will break out (most Un*x boxes
will core dump for a segmentation violation; dos boxes will print the
interrupt vector table until it finds a zero byte, etc).

Consult the FAQ for more information on using (or rather, `not' using) the
#define'd symbol `NULL'.
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Marshall Cline
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Marshall P. Cline, Ph.D. / Paradigm Shift, Inc / 65 N Main St/Norwood, NY 13668
cline@sun.soe.clarkson.edu / 315-353-4585 / FAX: 315-353-6100

govind@miller.cs.uwm.edu (Govind Seshadri) writes:

>Is it valid to use the NULL constant with cout ?
> Ex:  cout << NULL  ;
>I have seen that this has the  same effect as
>      cout << ""   ;
>on all the  C++  compilers  I have used.
>
>Or could this be a *bug*  in  these  compilers ??

I guess you shouldn't do this, since it is undefined what happens.

On my system NULL is defined (in iostream.h or in something it
includes) as just 0 and cout << NULL is equivalent to cout << 0 (and
this to cout << "0").  If your system defines NULL as (char*)0 (which
would be wrong for ANSI C but maybe your compiler is old) then the <<
operator for strings is called and if location 0 in your address space
contains a readable null byte it might print the empty string.

In ANSI C it is legal to define NULL as (void*)0, and if this is legal
in C++ too, it should print something like 0x0.

Finally note that different "standard" header files may contain
different definitions of NULL, and many user-supplied library files
*also* define it (if not already defined), so it may be a little hard
to find out how NULL is defined in your case, unless your compiler has
the equivalent of "CC -E" (show source after preprocessing).

--Guido van Rossum, CWI, Amsterdam <guido@cwi.nl>
"One's never alone with a rubber duck."

govind@miller.cs.uwm.edu (Govind Seshadri) writes:


>Is it valid to use the NULL constant with cout ?
> Ex:  cout << NULL  ;
>I have seen that this has the  same effect as
>      cout << ""   ;
>on all the  C++  compilers  I have used.

I can't believe this is the case.

NULL is usually defined to be a literal 0 in C++.  The expression
 cout << NULL
then becomes
 cout << 0
This calls ostream::operator<<(int), which should print the character zero.

It is possible, but unlikely, that NULL is defined as (void*)0,
as it usually is in Standard C.  In that case the expression calls
ostream::operator<<(void*), which should print out whatever a null
pointer looks like, typically "0x0".

Finally, the expression
 cout << ""
must call ostream::operator<<(const char*), which prints nothing,
since the string is empty.
--

Steve Clamage, TauMetric Corp, steve@taumet.com